Migration is another case of stranding metadata. And since
snapshot metadata is arbitrarily large, there's no way to
shoehorn it into the migration cookie of migration v3.
* src/qemu/qemu_driver.c (qemudDomainMigratePerform)
(qemuDomainMigrateBegin3, qemuDomainMigratePerform3): Add
restriction.

Yuck. I just noticed that this patch adds a migration restriction in
qemu_driver, but we already have migration restriction in _two_ other
places in qemu_migration.c. I'll refactor this to put all migration
restrictions into a single site, under qemuMigrationIsAllowed, to cut
down on the duplication.